Add link to api.coala.io
Add link to api.coala.io

Replace outdated Developers toc with link to
api documentation

Closes https://github.com/coala/documentation/issues/342

coala/documentation,kshitijkg01/documentation,Asnelchristian/documentation,rhemon/documentation,coala/documentation,coala/documentation,Warewolf96/documentation,AnishHota/documentation